How to prevent disconnection between two objects, while rigging?
For non-stretchy leg you should duplicate the foot bone and lets name it Target. The target bone has no parent and is the target of IK constraint. The foot bone is connected to the leg and copies the rotation of target bone with constraint.
Here is how it should look: 



I think you dont need the heel bone but if you do you can leave it parented to the Foot bone. Its also good to uncheck Deform for Target bone since it will only control the ik and not deform anything.



For the stretchy-leg set stretch of Leg and chains bones to 1:



But you will not end up with a much pleasing result, the stretchy rig is much more complicated.

If you need to implement also reverse-foot rig or you want to have more advanced stretchy leg you can ask a question specific to that or try the rigify addon, it will do all the work for you.